Sofc interconnect barriers and methods of making same using masks

ABSTRACT

A novel method to produce thin films spatially disposed on desired areas of workpieces is disclosed. Examples of include the formation of a yttria stabilized zirconia (YSZ) film formed on a desired portion of a stainless steel interconnect for solid oxide fuel cells by Atomic Layer Deposition (ALD). A number of methods to produce the spatially disposed YSZ film structures are described including polymeric and silicone rubber masks. The thin film structures have utility for preventing the reaction of glasses with metals, in particular alkali-earth containing glasses with ferritic stainless steels, allowing high temperature bonding of these materials.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. Utility Continuation application takingpriority from U.S. Provisional application No. 62/095,426, filed Dec.22, 2014, and from U.S. Utility application Ser. No. 14/975,810, filedDec. 20, 2015, both herein incorporated by reference.

BACKGROUND OF THE INVENTION References

-   W. N. Liu, X. Sun, E. V. Stephens, and M. A. Khaleel, “Investigation    of Performance of SCN-1 Pure Glass as Sealant Used in SOFC,” DOE    Topical Report PNNL-19407, Pacific Northwest National Laboratories,    March, 2010.-   R. N. Singh, “Innovative Self Healing Seals for Solid Oxide Fuel    Cells (SOFC),” 12th Ann. SECA Workshop, Pittsburgh, Pa., Jul. 28,    2011.-   J. W. Stevenson, G. G. Xia, S. P. Choi, Y. S. Chou, E. C.    Thomsen, K. J. Yoon, R. C. Scott, X. Li, and Z. Nie, “Development of    SOFC Interconnects and Coatings,” 12th Annual SECA Workshop,    Pittsburgh, Pa., Jul. 26-28, 2011.

The present invention relates to the formation of barrier layers onstainless steel substrates to reduce reaction between glass seals andthe substrate.

Solid oxide fuel cells (SOFC) represent a class of devices that converthydrogen in various forms to electricity using electrochemical reactionwith oxygen. The waste comprises water. SOFC's one of a growing numberof clean energy technologies. A critical reliability risk originates inthe glass seals that connect individual cells within the fuel cellstack. Each SOFC comprises an anode, electrolyte, and cathode. Anodesare typically Ni containing cermets, electrolytes are typically yttriastabilized zirconia, or related oxygen conductors, and cathodestypically are lanthanum-strontium based ceramics, including lanthanumstrontium manganite and lanthanum strontium iron cobaltite. State of theart fuel cell stacks utilize ferritic stainless steel sheets asinterconnect plates and manifolding between the fuel and air chambers,with mechanical bonding between plates using alkali earth containingsilicate glass formulations.

Despite generally favorable behavior, the glass/metal seal has onedistinct problem that must be overcome for long-term reliability. Duringoperation at elevated operating temperatures (650-850° C.), alkali-earthcomponents of the glass (e.g., Ba, Sr, and Ca) react with Cr in thestainless steel to form intermediate phases, e.g., SrCrO₄. As thisoccurs, the alkali-earth component of the glass is depleted viadiffusion, leading to formation of voids in the glass, whichsubsequently coalesce to cause leaks in the hermetic seal. (A previousinterconnect barrier layer scheme, utilizing solely aluminide basedbarriers, proved ineffective at mitigating this issue.) It has beendetermined that ytrria stabilized zirconia (YSZ) does not react withstainless steel or the glass, suggesting that it should be an excellentdiffusion barrier to prevent reactions between the sealing glass and thestainless steel interconnect.

Roughness of the steel interconnect presents a significant challenge tobarrier formation. Pacific Northwest Laboratories has recommended sandblasting to remove the mill scale and provide a reliable surface fromwhich the protective chromium oxide does not spall at elevatedtemperature. The surface of the steel is quite rough with numerous smallprotrusions, and with areas that may have re-entrant angles with respectto the surface. The ideal barrier would be thin, dense and fullycontinuous over such a surface and free of defects so that surface anddefect driven diffusion pathways are minimized. Regardless of thesurface treatment, the surface of the stainless steel is microscopicallyrough due to scratches, abrasion, and the like.

Numerous thin and thick film methods are known, but most have limitedability to provide a highly dense conformal coating over rough surfaces.Wet methods (e.g., sol-gel, pastes or screen printing) require sinteringand are non-conformal. Achieving high density is also a challenge withthese methods, which is exacerbated with ZrO₂ due to high sinteringtemperatures (˜1700° C.). Plasma or thermal sprays result in apolycrystalline, splat-like microstructure with poor conformality.Evaporation and sputtering have no and limited conformality,respectively. MOCVD offers a somewhat higher degree of conformality butrequires high temperatures. Accordingly, it would be a significantlyadvantageous improvement to produce a highly dense, conformal barrierlayer.

It is noted that while SOFC interconnects represent one applicationwhere such a barrier may be advantageously used, many other similarapplications exist, for example, alloys exposed to high temperaturecorrosive environments such as turbine blades.

SUMMARY DISCLOSURE OF INVENTION

The present invention relates to the fabrication of highly dense barriercoatings on ferritic stainless steels to reduce reaction with glassescontaining alkali-earth elements.

In one aspect, the invention relates to the use of atomic layerdeposition (ALD) to deposit a uniform layer of zirconia based ceramicfilm on ferritic stainless steel.

In another aspect, the invention relates to the use of zirconia basedceramic films fabricated in such a manner as a reaction blocking layerwith alkali-earth containing glasses.

In another aspect, the invention relates to the use of masking torestrict deposition of the barrier layer to desired areas on theworkpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a solid oxide fuel cell interconnect structure.

FIG. 2 is a cross-sectional electron micrograph image of a YSZ film onferritic stainless steel produced by atomic layer deposition (ALD).

FIG. 3 is a schematic of the interconnect-glass interface with a barrierlayer.

FIG. 4 is a comparison of reaction zone thicknesses in diffusion couplesof ferritic stainless steel and alkali-earth containing glass, with andwithout YSZ barriers after annealing at 850° C. for 260 hours.

FIG. 5 is a schematic of a masking method to prevent deposition ofbarrier layer in a region of the substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the fabrication of zirconium oxidebased ceramic barrier films on stainless steel by ALD with acomplimentary method of limiting deposition of the film to desiredareas.

FIG. 1 shows a schematic of a portion of an SOFC stack. The individualSOFC 1 is attached to the interconnect plate 2. Other ferritic stainlesssteel plates form gas manifolds 3 in proximity to the SOFC. The platesare mechanically bonded using alkali earth containing silicate glassformulations to provide a hermetic seal 4. The glass formulations remainvitreous at operating temperatures to provide mechanical compliance, andcan therefore react with the metallic plates. Ferritic stainless steeland other metallic substrate and accompanying glass or other reactivesolid formulations have been carefully engineered for oxidationresistance and the ability to accommodate thermal expansion,respectively. Ferritic stainless steels are characterized by a bodycentered cubic crystal structure, 10.5-29 atomic % chromium alloying,low nickel content (<5 atomic %) and are used in high temperaturecost-sensitive applications like catalytic converters. Examples includeAmerican Iron and Steel Institute (AISI) 440 and 441 (manufactured byAllegheney-Ludlum), Crofer APU22, Hitachi ZMG232L, and Sandvik SanergyHT 441. Viscous seal glasses include SEM-CON SC-1 and the like.

In one aspect, the invention relates to the use of ALD to depositzirconium oxide based films. Alloying additions to the zirconia filmsinclude fluorite phase stabilizing elements such as yttrium (Y) andscandium (Sc). These stabilizing elements may be added in ranges from 4mole percent to 8 mole percent for Y₂O₃, for example, to the zirconia inorder to stabilize the zirconia over a wider temperature range. Gradingof the YSZ through the thickness of the film may take place, with someregions having lower concentrations of the stabilizing element and otherregions having higher concentrations of the stabilizing element.

Using ALD, interlayers may also be added to the zirconium oxide basedfilms, for example, aluminum oxide (Al₂O₃), forming a nanolaminatestructure of alternating sublayers of aluminum oxide and zirconia,either or both materials of which may be doped with additionalstabilizing elements. The nanolaminate may be a bilayer, having just twolayers of different materials, or may have many alternating sublayers toform the overall thin film barrier structure. Grading of any sublayerwithin the nanolaminate may take place, with some regions of thesublayer having lower concentrations of the stabilizing element andother regions having higher concentrations of the stabilizing element.Use of one or more aluminum oxide sublayers in the nanolaminate may havebenefits including, but not limited to, adhesion, nanolaminatestabilization, and diffusion prevention.

The oxide ALD process uses pulses of a metal cation containing precursorand an oxidizing agent. The pulses are separated by pulses of inert gasto prevent reaction in the gas phase. A series of pulses characterizedby a precursor dose pulse, purge pulse, oxidizer pulse, and purge pulseis known as an ALD cycle. The process may be carried out under constantflow, or the gas flow may be stopped at intervals during dosing to allowdiffusion of species.

In the deposition process, each ALD metal cation layer may use a mixtureof precursors to produce multielement films, such as in the case of thepreviously mentioned yttria stabilization of zirconia, or differentratios of pulses may be used to produce a final film which containsmultiple elements. In this latter case, where different ratios of pulsesare used, one might, for example, use 9 pulse cycles of a zirconiumprecursor and oxidizer followed by 1 pulse cycle of yttrium precursorand oxidizer to produce a film containing approximately 10% of yttria inzirconia. Partial saturation of metal cation layers, each cycle with orwithout full reaction with oxidizers, may also be used to allow thiscomposition mixture with fewer overall cycles. A heat treatment oranneal may then be used to interdiffuse the layers and produce a desiredoverall mixture such as YSZ.

Note that a barrier layer on a metallic substrate may be directly incontact with the metallic substrate material, or may be over otherlayers which may be present on the substrate for various reasons suchas, but not limited to, adhesion, nucleation, thermal expansioncoefficient matching or crystallization of the barrier layer.

Metalorganic precursors for yttrium, scandium and zirconium include anumber of metalorganic compounds, including ketonates, iminates,alkoxides, amides, amidinates, guanidinates, and cyclopentadienyls. Ingeneral, many of these compounds are useful for ALD. Specifically,amides of Zr show excellent reactivity with water as an oxidizing agent.Useful amide sources for Zr include tetrakisdimethylamido Zr (TDMAZ),tetrakisdiethylamido Zr (TDEAZ), and tetrakisethylmethylamido Zr(TEMAZ). Yttrium and scandium sources with good reactivity for waterinclude triscyclopentadienyl Y (Y(Cp)₃), trismethylcyclopentadienyl Y(Y(Me-Cp)₃), trispropylcyclopentadienyl Y (Y(Pr-Cp)₃),triscyclopentadienyl Sc (Sc(Cp)₃), trismethylcyclopentadienyl Sc(Sc(Me-Cp)₂), and trisproylcyclopentadienyl Sc (Sc(Pr-Cp)₂). Otheroxidants may include ozone or oxygen plasma.

ALD may be carried out with solid or liquid sources held in bubblersthrough which a carrier gas is flowed to convey the source to thedeposition chamber. The sources may also be dissolved in an organicsolvent as individual sources or combined together. Key criteria of asolvent system are (1) high boiling point to reduce the chance of flashoff of the solvent, (2) high solubility for the compound, (3) low cost.Useful hydrocarbon solvents may include, for example: octane, decane,isopropanol, cyclohexane, tetrahydrofuran, and butyl acetate or mixturescomprising these and other organic solvents. Lewis base adducts may alsobe incorporated as additions to the solvent(s) for beneficial effects onsolubility and to prevent possible oligimerization of the precursormolecules. Examples of useful Lewis Bases include polyamines polyethers,crown ethers, and the like. Pentamethylenediamine is a one example of apolyamine. Examples of polyethers include various glymes such as mono-,di-, tri-, and tetraglyme.

Turning to the deposition process, we note that most ALD processesexhibit what is known as an ALD window with respect to temperature. Inthis temperature range, growth of the film is surface mononlayersaturation limited. The practical result is that deposition (thickness)per ALD cycle is the same, as long as sufficient material is provided tothe surface. After saturation, further supply of material in the gasphase does not increase growth per cycle. The primary objective in thepresent invention is the formation of a film of as conformal in natureas possible, i.e., with a uniform thickness over asperities.Crystallinity is also preferred. The upper end of the ALD windowtemperature range offers a good mix of conformality and the potentialfor good crystallinity. Post-deposition annealing in an oxygencontaining atmosphere may also be used to promote crystallinity in thefilm.

The deposition system may have an automated throttle valve that allowspressure to be controlled independently of flow. In this way, residencetimes can be manipulated more directly. The hot-wall type reactor is onetype of reactor that may be used to deposit the subject films.Alternatives include batch hot-wall reactor or warm-wall showerhead typereactors.

Process conditions favorable for ALD of zirconia based films are in thetemperature range of 150−250° C. with pressures in the range of 1-5Torr. Surface preparation (termination) can be very important in ALD.Pre-treatments to promote uniform nucleation include aqueous acids/basescompatible with the substrate and that result in —H or —OH terminationof the substrate surface.

Embodiments for ALD of zirconia based films on ferritic stainless steelinclude the following examples.

Example 1

A yttria stabilized zirconia film is deposited on ferritic stainlesssteel using TDEAZ and Y(Cp)₃ at 220° C. Reactor pressure is 2 Torr. Dosetimes for TDEAZ, Y(Cp)₃ and water vapor are 5 seconds. Purge times are10 seconds. Nitrogen is used to transport the precursors to thedeposition chamber and as the inert purge. A number of ALD cycles areused to deposit the film, with a ratio of 6 Zr cycles to 1 Y cycle.Interspersed in the ALD cycles are cycles of Al₂O₃ deposited usingtrimethylaluminum (TMA). The overall film thickness is between 5 and 500nm.

Example 2

A yttria stabilized zirconia film is deposited on ferritic stainlesssteel using TDEAZ and Y(Me-Cp)₃ at 230° C. Reactor pressure is 1 Torr.Dose times for TDEAZ, Y(Me-Cp)₃ and water vapor are 5 seconds. Purgetimes are 10 seconds. Nitrogen is used to transport the precursors tothe deposition chamber and as the inert purge. A number of ALD cyclesare used to deposit the film, with a ratio of 6 Zr cycles to 1 Y cycle.A hydroxide terminating treatment is used before ALD. The ferriticstainless steel has a Cr₂O₃ surface layer that is exposed to dilutenitric acid for 10 seconds at room temperature. The surface is rinsed inwater and then exposed to a mixture of sulfuric acid and 30% hydrogenperoxide-water in a 3:1 ratio for 10 seconds. The surface is then rinsedand dried. The Y dose is applied first to the substrate, followed by Zr.The overall film thickness is between 5 and 500 nm.

Example 3

A yttria stabilized zirconia film is deposited on ferritic stainlesssteel using TDMAZ and Y(Me-Cp)₃ at 230° C. Reactor pressure is 1 Torr.Dose times for TDMAZ, Y(Me-Cp)₃ and water vapor are 5 seconds. Purgetimes are 10 seconds. Nitrogen is used to transport the precursors tothe deposition chamber and as the inert purge. A number of ALD cyclesare used to deposit the film, with a ratio of 6 Zr cycles to 1 Y cycle.A dense, highly conformal YSZ film deposited on ferritic stainless steelusing 798 ALD cycles is shown in the SEM micrographs of FIG. 2. The filmthickness is approximately 80 nm.

In another aspect, the invention relates to the use of zirconia basedceramic films fabricated in such a manner as a reaction blocking layerwith alkali-earth containing glasses. In FIG. 3, a YSZ layer 5 isdisposed between the ferritic stainless steel 6 and an alkali-earthcontaining glass 7. A passivating chromium oxide layer 8 may be presenton the surface of the stainless steel 6. The YSZ film is produced byALD.

FIG. 4 shows the thickness of reaction zones between stainless steel andglass, where less reaction (a thinner layer) is better. After annealingat 850° C. for 260 hours, the extent of reaction of the stainless steelwith the glass is characterized by examining the width of the reactionzone by cross-sectional analysis of a polished section where the extentof reaction is shown for a glass-stainless steel diffusion couple with(A) and without a YSZ barrier deposited by ALD. The YSZ barrierdecreases the extent of reaction by a factor of 10. Differentcombinations of barrier layer materials, thickness, nanocompositemultilayer stacking, metallic substrate and reactive solid, as well asreaction temperature and time may vary the factor of reduction ofinter-reaction between the metallic substrate and the reactive solid. Itis desirable to achieve at least a reduction of a factor of 2, moredesirable to achieve a reduction of a factor of 5, and in a preferredembodiment achieve a reduction of a factor of 10 or more. The barriermay be useful for protecting other metals from reaction with deleteriouscations in applications outside of SOFCs.

In another aspect, the invention relates to the use of masking torestrict deposition of the barrier layer to desired areas on theworkpiece. It is well known that ALD processes coat workpieces quiteuniformly and typically over all exposed surfaces. It is therefore agreat advantage to devise a scheme by which only a portion of thesurface is coated, specifically in a desired area. For SOFCinterconnects, the desired area is the perimeter of the interconnectwhere the glass seal is formed. It is desired not to coat the centralarea of the interconnect where electrical conductivity is needed. Athermally stable mask is used. The low thin film deposition temperatureachievable via use of ALD in this invention is particularly advantageousin allowing use of inexpensive, easily shaped, flexible and reusablemask materials such as elastomers to carry out this masking process.

Example 4

A self-adhesive polyimide film is applied to the substrate prior todeposition of YSZ by ALD. After deposition, the polyimide film is peeledoff.

Example 5

FIG. 5 shows a sandwich structure of interleaved multiple ferriticstainless steel plates 9 and silicone rubber sheets 10. This structureis placed in a clamping mechanism and compressed so that the masks arein intimate contact with the substrates. An ALD process for YSZ iscarried out on the assembly at 230° C. for 800 cycles. After deposition,the silicone is removed. The area of each sheet 9 which was masked bysilicone rubber sheet 10 does not have a YSZ coating, as it wasprotected from reaction with the precursors or oxidizer during the ALDprocess.

Multiple substrates and masks may be stacked to allow many substrates tobe coated simultaneously. The masks may be continuous sheets of siliconerubber or gasket like sheets where a perimeter seal protects andinterior area. The masks may be clamped outside of the ALD process(ex-situ) or inside the ALD process chamber (in-situ) by suitablemechanisms such as a camshaft actuated via a rotary vacuum feed-through.Mask materials may also be applied or attached to the substrate asliquids or viscous liquids by dispensing or flowing the mask materialonto the substrate, which case the shaping of the mask structure may becarried out by methods such as lithography, stencils, stamps, etc.

The subject invention may be embodied in the forgoing examples that areby no means restrictive, but intended to illustrate the invention.

What is claimed is:
 1. A masking apparatus, comprising a mask material which is both flexible and thermally stable in the temperature range of 50-300 degrees Celsius, the mask material being removably attached to a substrate in order to prevent atomic layer deposition of a thin film on the substrate on the region of the substrate covered by the mask material.
 2. The masking apparatus of claim 1, wherein the mask material is a polymer.
 3. The masking apparatus of claim 1, wherein the mask material is silicone rubber.
 4. The masking apparatus of claim 1, wherein the mask material is polyimide.
 5. The masking apparatus of claim 1, wherein the mask material is held in place by clamping.
 6. The masking apparatus of claim 1, wherein the clamping is accomplished by applying a differential pressure above and below the mask material.
 7. The masking apparatus of claim 1, wherein multiple masks and substrates are stacked and clamped as a group during atomic layer deposition, thereby preventing atomic layer deposition of the thin film in defined regions of each substrate.
 8. A masking method, comprising choosing a mask material which is both flexible and thermally stable in the temperature range of 50-300 degrees Celsius, shaping the mask material into a shape which can conformally cover and thereby mask out a region of a substrate whereupon it is desirable not to deposit a thin film, attaching the shaped mask material to the substrate, placing the masked substrate into a deposition chamber, and depositing the thin film by atomic layer deposition, thereby covering at least part of the unmasked substrate with the thin film.
 9. The masking method of claim 8, wherein attaching the shaped mask material to the substrate is accomplished by applying a differential pressure above and below the mask material.
 10. The masking method of claim 8, wherein after depositing the thin film by atomic layer deposition, further removing the shaped mask material and incorporating a portion of the substrate into the structure of a solid oxide fuel cell.
 11. The masking method of claim 8, wherein after depositing the thin film by atomic layer deposition, further removing the shaped mask material and bringing a portion of the substrate which has been covered by the thin film into contact with a vitreous reactive solid. 